1. Field of the Invention
The present invention relates to a device for performing phase difference detection type autofocus. Particularly, the present invention relates to a focus detecting (hereinafter also referred to as AF) optical device (AF sensor) used in a digital camera or an analog (silver halide) camera or the like.
2. Description of the Related Art
FIG. 2 shows a layout of a focus detecting optical device for performing conventional area type phase difference detection type autofocus. This area type phase difference detection type AF sensor is disclosed in Japanese Patent Application Laid-Open No. H11-019867 and Japanese Patent Application Laid-Open No. 2005-109370 which are both commonly assigned with the present application.
In FIG. 2, a chip is formed on the same semiconductor substrate by using a CMOS process or the like, and is composed of a sensor circuit block 201, an analog circuit block 202, and a digital circuit block 203. Each of the pixels of area sensors 204, 205, 206 and 207 has a photoelectric conversion element such as a photo diode, and an imaging region for capturing an image of an object is formed by arranging these pixels in a two dimensional manner. Because the phase difference detection type AF sensor performs focus detection by detecting a phase difference between two objects, imaging regions including an A image (standard portion) and a B image (reference portion) are required.
The B image of the area sensor 1′ 205 is placed symmetrically to the A image of the area sensor 1 204 relative to a vertical line which passes through an optical center 214 (also referred to as a center of the sensor). The B image of the area sensor 2′ 207 is also placed symmetrically to the A image of the area sensor 2 206 relative to a horizontal line which passes through the optical center 214. The number of pixels and the pixel size of the area sensor 1 are same as those of the area sensor 1′. The pixel size is 13.6 um×101.6 um, and the pixels are arranged in an array of 56 pixels×18 rows. The number of pixels and the pixel size of the area sensor 2 are same as those of the area sensor 2′. The pixel size is 13.6 um×73.6 um, and the pixels are arranged in an array of 45 pixels×42 columns. In addition, reference number 208 in the analog circuit block denotes an AGC circuit for automatically controlling the gain of a signal output circuit based on an accumulation period of the AF sensor. Reference numeral 209 denotes a signal amplifying circuit for amplifying a photoelectric conversion signal of the AF sensor to output. Further, reference numeral 210 denotes a power supply circuit for generating a reference voltage required to drive the sensor. Furthermore, reference numeral 211 in the digital circuit block denotes a SRAM for holding the accumulation period information, a gain setting value and the like of the AF sensor. Reference numeral 212 denotes a multiplexer circuit for selecting an analog signal to output for monitoring, and reference numeral 213 denotes a timing generator and I/Os.
Although the above mentioned focus detecting optical device for performing phase difference detection type autofocus is used for multi detection point high speed high precision autofocus in a wide region, there appears a problem of the cost increase due to the increase of the chip area. This is a problem specific to the area type AF sensor, which problem has not appeared in a line type AF sensor in which there is the small number of pixels and which has relative free arrangement in the case where the pixel area is increased. In FIG. 2, for example, a case that autofocus performance is increased under a low (dark) intensity condition is supposed. In this case, if the respective areas of the photo diodes in the area sensor 1 and the area sensor 1′ are uniformly increased for the purpose of improving the sensitivity, these sensors do not fit into the current chip, thus increasing the chip area. Therefore, the cost of the focus detecting optical device has been increased and these devices have not been put into practical use.